Print this pageThe Role of Faulting in the Localisation of Diagenetic Cements and Porosity in a UK North Sea Oil Field
GUSCOTT, SIMON C., STUART D. BURLEY, and JAMES D. MARSHALL
Upper Jurassic reservoir sandstones of the Outer Moray Firth Basin are characterised by a relatively simple burial diagenetic history. However, proximal to a fault zone along the south-west flank of the Witch Ground Graben, a late fracture-hosted mineral assemblage is recognised, comprising ankerite, kaolinite, barite, galena and calcite. This mineral assemblage post-dates extensive burial diagenesis and is contemporaneous with on-structure hydrocarbon migration.
Microthermometric and isotopic analyses of the fracture-hosted mineral cements reveal large salinity fluctuations and an associated carbon isotope anomaly in the palaeo-pore waters proximal to the fault zone, such anomalies are not observed away from the fault. These data provide evidence for the presence of extraformational fluids within the reservoir sandstones proximal to the fault. A highly saline, isotopically heavy fluid is envisaged to have migrated in to the reservoir sandstones from the near-by Zechstein Formation, whilst a hydrocarbon-rich fluid, originating in the Kimmeridge Clay Formation, also entered the reservoir structure. The mixing of these two fluids, and their interaction with the connate waters, resulted in the precipitation of the observed fracture-hosted mineral assemblage.
The vertical migration of brines from the Zechstein into the Jurassic reservoir sequence provides unequivocal evidence that faults have played a significant role in the focussing of subsurface fluid flow and have had a tangible effect on reservoir quality of sandstones proximal to faults.